1 USE OF 500 GRADE STEEL IN THE DESIGN OF REINFORCED CONCRETE SLAB Prof. M. Shafiul Bari, Ph.D Department of Civil Engg., BUET Introduction There is growing interest within the reinforced concrete industry in using higher strength reinforcing steel for certain applications. This interest is driven primarily by relief of congestion; particularly in buildings assigned a high seismic design category. There are also other areas where high strength bar can help improve construction efficiencies, or - combined with high strength concrete - allow reinforced concrete to be used in more demanding applications. Today, the vast majority of concrete design and construction uses Grade 60 steel, with occasional but increasing use of Grade 75 (Ref 1). Several different grades of steel may be used for large projects, with a minimum grade for ordinary tasks and higher grades for more demanding ones. Cost increases generally for higher grades, so some feasibility studies must be made to see if the better steel in smaller quantities is really cheaper than a larger quantity of a lower grade. Actually, higher grades are often used to permit smaller concrete members, relating to the space problems for placement of the reinforcement. Even though the steel ordinarily constitutes only a few percent of the total volume of reinforced concrete, it is a major cost factor. This includes the cost of the steel, the forming of the deformed bars, the cutting and bending required, and the installation in the forms. A cost- saving factor is usually represented by the general attempt to use the minimum reinforcement and the most concrete, reflecting typical unit costs for the two materials. The use of different grades of steel in different building Codes and Standards and various design provisions of codes for the design of different structural members are discussed in details in the following articles. Design/material Provision in Bangladesh National Building Code (BNBC-93) (Ref 2,3) Deformed reinforcing bar shall conform to one of the following specification BDS 1313, ASTM A615, ASTM A706, BS 4461 (Art 5.3.2.1) Deformed reinforcing bar with a specified yield strength fy exceeding 410 N/mm2 shall be permitted, provided fy shall be the stress corresponding to a strain of 0.35 percent and the bars otherwise conform to one of the ASTM specification listed above. (Art 5.3.2.2) Yield strength of the reinforcement fy shall not be taken more than 550 N/mm 2 . (Art 6.1.2.5) The ultimate tensile strength of any bar shall be greater than the actual yield strength measured in the tensile test by at least 15% for Grades 250, 275, 350 and 400 and at least 10% for Grade 500, as per Art.8 of BDS 1313: 1991 (Ref 3).
Transcript
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USE OF 500 GRADE STEEL IN THE DESIGN
OF
REINFORCED CONCRETE SLAB
Prof. M. Shafiul Bari, Ph.D
Department of Civil Engg., BUET
Introduction
There is growing interest within the reinforced concrete industry in using higher strength
reinforcing steel for certain applications. This interest is driven primarily by relief of
congestion; particularly in buildings assigned a high seismic design category. There are also
other areas where high strength bar can help improve construction efficiencies, or - combined
with high strength concrete - allow reinforced concrete to be used in more demanding
applications. Today, the vast majority of concrete design and construction uses Grade 60
steel, with occasional but increasing use of Grade 75 (Ref 1).
Several different grades of steel may be used for large projects, with a minimum grade for
ordinary tasks and higher grades for more demanding ones. Cost increases generally for
higher grades, so some feasibility studies must be made to see if the better steel in smaller
quantities is really cheaper than a larger quantity of a lower grade. Actually, higher grades are
often used to permit smaller concrete members, relating to the space problems for placement
of the reinforcement.
Even though the steel ordinarily constitutes only a few percent of the total volume of
reinforced concrete, it is a major cost factor. This includes the cost of the steel, the forming of
the deformed bars, the cutting and bending required, and the installation in the forms. A cost-
saving factor is usually represented by the general attempt to use the minimum reinforcement
and the most concrete, reflecting typical unit costs for the two materials.
The use of different grades of steel in different building Codes and Standards and various
design provisions of codes for the design of different structural members are discussed in
details in the following articles.
Design/material Provision in Bangladesh National Building Code (BNBC-93) (Ref 2,3)
Deformed reinforcing bar shall conform to one of the following specification BDS 1313,
ASTM A615, ASTM A706, BS 4461 (Art 5.3.2.1)
Deformed reinforcing bar with a specified yield strength fy exceeding 410 N/mm2 shall be
permitted, provided fy shall be the stress corresponding to a strain of 0.35 percent and the bars
otherwise conform to one of the ASTM specification listed above. (Art 5.3.2.2)
Yield strength of the reinforcement fy shall not be taken more than 550 N/mm2. (Art 6.1.2.5)
The ultimate tensile strength of any bar shall be greater than the actual yield strength
measured in the tensile test by at least 15% for Grades 250, 275, 350 and 400 and at least
10% for Grade 500, as per Art.8 of BDS 1313: 1991 (Ref 3).
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Limitations on materials for Seismic design:
Concrete in members resisting earthquake induced forces, fc’ shall be not less than 20 N/mm2
And reinforcing steel in members resisting earthquake induced flexural and axial forces in
frames and wall boundary members shall comply with ASTM A706, ASTM A615 and BDS
1313. Reinforcement with fy = 275N/mm2 and fy = 410N/mm2 are allowed in these
members if the following two conditions are satisfied.
.
Actual fy ≤ specified fy + 125 N/mm2
Actual ultimate tensile stress
---------------------------------- ≥ 1.25
actual fy
Provision for Materials in ACI Code (Ref 4)
The ACI Code allows the deformed reinforcement as given in Art 3.5.3 of the Code.
Deformed reinforcing bars shall conform to the requirements for deformed bars in one of the
following specifications, (Art 3.5.3.1)
(a) Carbon steel: ASTM A615; (Ref 5)
(b) Low-alloy steel: ASTM A706; (Ref 6)
(c) Stainless steel: ASTM A955;
(d) Rail steel and axle steel: ASTM A996. Bars from rail steel shall be Type R.
ASTM A615/ A615-96a specifies bars of three minimum yield levels: namely, 40,000 psi
(300 MPa), 60,000 psi (420 MPa), and 75000 psi (520 MPa), designated as Grade 40 [300],
Grade 60 [420] and Grade 75 [520] respectively. The material, as represented by the test
specimen, shall conform to the requirements for tensile properties prescribed in Table 1
Table 1 : Summary of minimum ASTM strength requirements
Product ASTM
Specification Designation
Minimum Yield
Strength, psi (MPa)
Minimum Tensile
Strength, psi
(MPa)
Reinforcing bars A615 Grade 40
Grade 60
Grade 75
40,000 (280)
60,000 (420)
75,000 (520)
60,000 (420)
90,000 (620)
100,000 (690)
A706 Grade 60 60,000 (420)
[78,000 (540)
maximum]
80,000 (550)a
A996 Grade 40
Grade 50
Grade 60
40,000 (280)
50,000 (350)
60,000 (420)
60,000 (420)
80,000 (550)
90,000 (620)
a. But not less then 1.25 times the actual yield strength.
Deformed reinforcing bars shall conform to one of the ASTM specifications listed in 3.5.3.1,
except that for bars with fy exceeding 60,000 psi, the yield strength shall be taken as the
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stress corresponding to a strain of 0.35 percent. The values of fy and fyt used in design
calculations shall not exceed 80,000 psi, except for prestressing steel and for transverse
reinforcement (Art 3.5.3.2).
Limitation on Materials:
A minimum specified concrete compressive strength fc' of 3000 psi and a maximum specified
reinforcement yield strength fy of 60,000 psi are mandated. These limits are imposed as
reasonable bounds on the variation of material properties, particularly with respect to their
unfavorable effects on the sectional ductilities of members in which they are used. A decrease
in the concrete strength and an increase in the yield strength of the tensile reinforcement tend
to decrease the ultimate curvature and hence the sectional ductility of a member subjected to
flexure.
Chapter 21 requires that reinforcement for resisting flexure and axial forces in frame
members and wall boundary element be ASTM A706 grade 60 low alloy steel, which is
intended for application where welding or bending, or both, are important. However, ASTM
A615 billet steel bars of grade 40 or 60 may be used in these members if the following two
conditions are satisfied.
Actual fy ≤ specified fy + 18,000 psi
Actual ultimate tensile stress
---------------------------------- ≥ 1.25
actual fy
The first requirement helps to limit the magnitude of the actual shears that can develop in a
flexural member above that computed on the basis of specified yield value when plastic
hinges form at the ends of a beam. The second requirements in intended to ensure steel with a
sufficiently long yield plateau.
In the “strong column-weak beam” frame intended by the code, the relationship between the
moment strengths of columns and beams may be upset if the beams turn out to have much
greater moment strengths than intended. Thus, the substitution of Grade 60 steel of the same
area for specified Grade 40 steel in beams can be detrimental. The shear strength of beams
and columns, which is generally based on the condition of plastic hinges forming at the ends
of the members, may become inadequate if the moment strengths of member ends should be
greater than intended as a result of the steel having a substantially greater yield strength than
specified.
ACI Code Provisions for Minimum Slab Thickness.
ACI code 9.5.2.1 specifies the minimum thickness of the non-pre-stressed one -way slabs
using Grade 60 reinforcement as given in Table 9.5(a).
For slabs without interior beams spanning between the supports and having a ratio of long to
short span not greater than 2, the minimum thickness shall be in accordance with the
provisions of Table 9.5(c) and shall not be less than the following values (Art 9.5.3.2)
(a) Slabs without drop panels .............................5 in.
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(b) Slabs with drop panels...................................4 in.
For slabs with beams spanning between the supports on all sides, the minimum thickness, h,
shall be as follows (Art 9.5.3.3)
(a) For αm equal to or less than 0.2, the provisions of 9.5.3.2 shall apply;
(b) For αm greater than 0.2 but not greater than 2.0, h shall not be less than
and not less than 5 in.;
(c) For αm greater than 2.0, h shall not be less than
and not less than 3.5 in.;
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Provision in British/European Standard BS-4449/prEN-100800 (Ref 7)
BS Standard contains provision for 3 steel grades such as B500A, B500B & B500C all of
500MPa characteristic yield strength, but different ductility characteristics. The code pr EN
10080 allows Grade 500 steel.
Provisions for Cast in Situ Solid Slabs in EN code.
For a solid slab, the absolute minimum thickness is |50 mm|. (Art 5.4.3.1)
The maximum spacing of the bars is as follows (Art 5.4.3.2).
For the principal reinforcement, |1.5h 350 mm|, where h denotes the total depth of
the slab;
For the secondary reinforcement, |2.5 h 400 mm|.
Provisions in Indian Standard IS 456 : 2000 (Ref 8)
The reinforcement shall be any of the following:
a) Mild steel and medium tensile steel bars conforming to IS 432 (Part 1).
b) High strength deformed steel bars conforming to IS 1786 (Ref 9)
c) Hard-drawn steel wire fabric conforming to IS 1566.
d) Structural steel conforming to Grade-A of IS 2062.
Proof stress, percentage elongation and tensile strength for all sizes of deformed bars/wires
determined on effective cross-sectional area (see 5.3) and in accordance with 8.2 shall be as
specified in Table 3.(Art 7.1)
TABLE 3 MECHANICAL PROPERTIES OF HIGH STRENGTH
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DEFORMED BARS AND WIRES
Sr PROPERTY GRADE
No
Fe 415 Fe 500 Fe 550
(1) (2) (3) (4) (5)
i) 0.2 percent proof stress/ 415.0 500.0 550.0
yield stress, Min, N/mm1
ii) Elongation, percent, Min, 14.5 12.0 8.0
on gauge length 5.65 I/AT
where A is the cross sectional
area of the test piece
iii) Tensile strength, 10 percent more 8 percent more 6 percent more
Min than the actual 0.2 than the actual 0.2 than the actual 0.2
